1. Field of the Invention
The present invention relates to a power control system, more specifically, to a power control system that shifts an operation mode of a sub system connected to a main system to a power-saving mode or a normal operation mode.
2. Description of the Related Art
Recently, power saving control of central processing unit (CPU) employed in an image forming apparatus is more sophisticated compared with the conventional one. For example, in a power-saving mode, by retaining operation information of CPU while reducing the power supply voltage to such a voltage with which the CPU itself is not operable, electric power consumption in the power-saving mode is saved.
In an image forming apparatus having a main system and a sub system, operation mode of the sub system is shifted to a power-saving mode by outputting a command for stopping supply of power to the subsystem, to a power supply control unit that supplies each sub system with power from the main system, thereby reducing supply voltage to the sub system.
However, in shifting to the power-saving mode, when there is a process under processing in the sub system, there arises a problem that the mode shifts to the power-saving mode before completion of the process. There is also a problem that a CPU cannot be activated because of low supply voltage even when an activation signal for activating the CPU is output to the CPU in the condition that the power supply voltage is reduced to such a voltage with which operation of the CPU for sub system control itself is disabled, in the power-saving mode of the sub system.
Japanese Patent Application Laid-open No. 2003-131766 discloses a technique that solves the problem by controlling a timing of shifting to the power-saving mode of the system, and a timing of outputting an activation signal for returning from the power-saving mode of the system by means of a timer.
However, in the conventional art, in conducting power supply control, power supply management is generally realized by using a sub microcomputer or a special device, however, use of such a sub microcomputer or a special device exclusively for the purpose of power supply management will result in increases in production cost and circuit scale and hence is undesirable from the view point of substrate implementation area.